1. Field of the Invention
The present invention generally relates to optical measurement techniques and, more particularly, to the detection of boundaries and thicknesses with very high resolution, especially in controlling manufacturing processes of material deposition, removal, and reaction and, most particularly, in the fabrication of integrated circuits.
2. Description of the Prior Art
Modern electronic, opto-electronic devices and the like are complex structures formed by many repeated steps of material deposition, reaction, modification (e.g. annealing), and removal. These processes are common to the fabrication of individual devices, integrated circuits and magnetic recording media (such as surfaces formed by sputtering) and apparatus. While typical dimensions of lateral features in such applications are frequently in the range of one micron, the thickness of layers is often far smaller than this scale, sometimes down to the range of 100 Angstroms. A thickness of 100 Angstroms corresponds to only some 50 atomic layers. It can be easily understood that precise measurement of the dimensions during processing is crucial to device performance and to the yield of the manufacturing process.
One of the critical general issues is the detection of a boundary during the removal of material. Material removal may occur by methods such as chemical etching or by a plasma process such as reactive ion etching. It is generally necessary to determine the time at which all of the desired material has been removed, but the underlying layer has not been etched significantly. This constitutes the so-called detection of endpoints. The etched material and the underlying material are frequently made of different compositions, such as silicon dioxide on a silicon layer. They may, however, differ primarily in their crystalline structure or doping level and not in their gross chemical composition. This poses particular challenges for the problem of endpoint detection. For example, in the manufacture of bipolar transistors the step of emitter opening involves etching an area of a deposited layer of polycrystalline silicon down to an epitaxial (single crystal) layer. This operation is typically performed by reactive ion etching which does not present any obvious means for real-time control. The consequences of improper etching are, however, significant. Underetching will cause a degradation of the gain of the transistor, while overetching will result in a degraded contact between the intrinsic and extrinsic base regions.
While some etching processes may provide selective removal of the overlayer and a slow rate of removal of the underlying material, this is not always attainable in practice. A real manufacturing process has numerous constraints, such as obtaining high etch rates and using relatively safe chemicals. The step may also require a strongly anisotropic etch rate. These conditions collectively mean that a highly selective etch may not be available. In this instance, the capability of monitoring the progress of the etching process during manufacturing becomes of critical importance.
A related issue is the determination of the thickness of layers of finite dimensions. This may be of relevance either in removing material to obtain a layer of a given thickness or in depositing material until a layer of a given thickness is reached. To cite one example, consider the gate insulator in a field effect transistor. The thickness of the insulating region will affect the switching voltage and the uniformity of the thickness will affect the uniformity of performance of transistors in an integrated circuit. The thickness of a layer formed by the reaction of two materials is also a quantity of interest. Examples of this situation occur in the formation of a silicon oxide layer by a chemical reaction with gas species containing oxygen and in the formation of a metal silicide on a silicon surface, an important process for forming contacts in semiconductor devices.
It should be noted that some reactions and modifications of surfaces will occur spontaneously such as oxidation of silicon at room temperature in the presence of oxygen and the formation of silicides. However, such spontaneous reactions typically will only occur to a small depth which is trivial for purposes of fabrication of any useful device. However, such processes can be made to continue in a controllable manner by imposing appropriate conditions such as high temperature. Accordingly, by reference herein to controllable surface reactions and modifications, it is intended to exclude reactions and modifications which occur spontaneously at relatively low temperatures and are not controllable in the sense in which a manufacturing process may be controlled.
Many techniques have been developed for examining the thickness and other properties of thin layers subsequent to processing. One of the important techniques for post-processing analysis is secondary ion mass spectrometry. This process involves detection of the material by mass spectrometric means as it is sputtered by an impinging ion beam. Although providing important and precise information on chemical composition and thicknesses of layers, this technique is destructive, since it destroys the sample under investigation. It is also not suitable for in-situ measurements even if its destructive character could be tolerated. Similar considerations apply for electron microscopy and direct mechanical measurements which are other familiar approaches for post-processing analysis. Other methods appropriate for surface analysis may give very high sensitivity. These include electron diffraction techniques (LEED and RHEED), electron energy loss spectroscopy, Auger electron spectroscopy, photoelectron spectroscopy, and so forth. These latter methods are, however, all restricted to high vacuum environments and cannot be used under realistic processing conditions.
It should be noted that all techniques which are not adaptable to in-situ process monitoring and destructive testing techniques, in particular, rely on trial-and-error development of an appropriate process and the repeatability of the process itself to form the desired structures. The trial-and-error process development increases cost of the process and reliance on repeatability reduces yield, particularly in devices in which endpoints and thicknesses are critical.
Non-destructive techniques for real-time, in-situ, analysis in processing environments are far fewer than those summarized above. Non-destructive techniques are typically optical, although in some cases electrical measurements may be appropriate, depending on the processing environment.
Electrical techniques usually depend on the measurement of the electrical resistance or the voltage dependence of the electrical resistance determined by passing a current through the layer either in a direction parallel or perpendicular to the surface layer. The analysis of such measurements may be quite complex, since the observed resistance depends on a variety of factors in addition to the layer thickness, such as the width of the layer, the influence of underlying structures, and the material properties. The measurements may also be strongly affected by factors such as the contact resistance and are clearly limited in application to materials with reasonable electrical conductively. Refinements in these techniques are capable of detecting slight nonlinearities in the relation between applied voltage and the induced current, that is, a voltage-dependent effective resistance. These approaches are particularly useful for identifying material defects, but do not necessarily improve the method for detecting boundaries and determining layer thicknesses. From the processing point of view, electrical measurements are also undesirable. Electrical measurements are generally invasive, requiring a current to be passed through the material under test and requiring contacts to be made to the device or region of the material under investigation. This imposes practical difficulties such as the provision of space for such contacts, in turn limiting potential integration density, and certain processing environments. Such practical limitations may render the approach entirely impractical for some devices and impossible in typical environments such as plasmas or liquid chemical etchants.
The above-described electrical testing techniques are exemplified by Heiber et al. U.S. Pat. No. 4,562,089, which relies on resistance measurements, and DiStephano et al, U.S. Pat. No. 4,496,900, which applies an alternating voltage to a region and detects defects by observing a second harmonic of the applied voltage frequency in the current response arising from a voltage dependent conductivity.
It should also be noted that measurement of conductivity characteristics is an effect within the bulk of the material as well as being affected by the geometry of the conductive region and ambient conditions such as the presence of a plasma or liquid chemical etchant. Therefore, the technique disclosed by Heiber is unlikely to be usable in typical manufacturing process environments and is clearly inapplicable to monitoring the formation of insulator structures and problems such as the emitter opening technique described above, both of which involve monitoring of an area generally parallel to a surface rather than a current path through the bulk of a material. For instance, a current path could become discontinuous while substantial material remained unetched or could remain continuous while overetching was taking place in other portions of the area.
The most broadly applicable methods for real-time, in-situ analysis are based on optics. Optical methods can often be adapted to widely varying environmental conditions such as are found in chemical and reactive-ion etching, plasma-enhanced vapor deposition, etc. The purely optical approaches usually also have the advantage of being non-destructive and non-invasive. Moreover, the methods can generally yield lateral resolution (to approximately 1 .mu.m) by focusing the relevant light beams or by means of imaging techniques.
For the most part optical methods for direct examination of surface structures involve a direct measurement of the reflectivity of the sample. Since optical radiation typically penetrates hundreds or thousands of atomic spacings (distances greater than 100 nm), it is difficult to obtain sensitivity on the level of one or a few atomic layers, which is the resolution desired for very precise control of processing. The sensitivity of reflectivity measurements can be enhanced in various ways, such as a judicious choice of the wavelength of the light used in the measurement. The most common refinement involves use of ellipsometry.
In this well-established technique, the polarization of the reflected light is analyzed. Since polarization can be measured with great accuracy with suitable instrumentation, the method can be highly sensitive to changes in surface properties and the thickness of material layers. The method has, however, significant practical limitations. The very small changes in polarization associated variation in film thickness by a few atomic layers can easily be masked by other effects arising from the bulk materials. These effects include the influence of slight temperature changes, the presence of strain, etc. Also, measurement in a manufacturing environment is difficult given the stringent requirements on geometric arrangement and geometric stability. Further the optical properties of windows, notably stress-induced birefringence, may significantly distort the measurements.
A different improved optical technique for determining layer thickness and the detection of boundaries is disclosed by Tien, U.S. Pat. No. 4,713,140. The technique disclosed by Tien relies upon the luminescence of direct band-gap semiconductors. This approach has a number of significant limitations, as well. First the method is suitable only for direct band-gap semi-conductors, thereby excluding silicon, metals, and insulators, the materials of greatest importance currently in the electronics industry. Moreover, the emitted luminescence emerges as weak incoherent radiation. Therefore, while this technique can be applied to a small region, a lens is necessary for collection of the luminescent radiation which presents severe difficulties since the lens must be positioned closely to the irradiated region to collect the radiation of interest. Inaccuracies of lens positioning and sensitivity of the process to ambient radiation conditions could greatly distort the measurement results. The broad band luminescence radiation, typically at red regions of the spectrum at wavelengths greater than 800 nm, also presents particular difficulty in discriminating from background radiation (be it ambient light, plasma or thermal radiation) typically present or at least difficult to exclude from semiconductor manufacturing processes. The same problems are associated with the application of other techniques based on the emission of incoherent light, such as surface light scattering and Raman scattering.
Again, it should be noted that reflection observations, including ellipsometry, are bulk effects of the material and, although the effects are relatively strong, are inherently limited in accuracy due to the penetration of illuminating radiation into the bulk of the material and the thickness of that bulk which contributes to the response. This inherent limitation is also true of the technique of Tien, described above.
In more general terms, all of the optical techniques applied prior to the invention disclosed herein are difficult or impossible to adapt to yield sensitivity to surfaces or interfaces of materials with a sensitivity on the level of a single atomic layer or spacing, i.e., a fraction of a nanometer. This difficulty is inherent to the large penetration depth of the optical radiation, which implies that the effects of bulk materials will be far stronger than that of a thin layer comprising a surface or interface region. While sensitivity can sometimes be enhanced, as indicated above, this usually places severe restrictions on the choice of materials or on the instrumentation. Further, the measurements are generally subjected to potential errors from slight changes in the properties of the bulk materials, as may occur from strain, temperature or temperature gradients, and the like, as well as limitations on optical access and potential errors imposed due to the above-described imperfections of windows and other means of providing optical access to the material surface during processing.
In summary, a particular challenge has continued to exist in developing methods suitable for non-destructive, non-invasive measurements with a sensitivity approaching that of a single atomic layer.
Certain non-linear optical effects are known in fields heretofore unrelated to material processing, in general, or semiconductor device manufacture, in particular. For instance, the SHG effect consists of the production of light at twice the frequency of a pump beam. The process can be considered as the combining of two photons of energy E to produce a single photon of energy 2E, i.e., the production of light of twice the frequency (or half the wavelength) of the pump radiation. This effect can also be generalized to the combining of photons of different energies, corresponding to different frequencies, as well, as will be pointed out below (referred to as wave-mixing or sum- and difference-frequency generation). However, in the interest of clarity, the invention will be explained principally in terms of the second harmonic generation effect.
The existence of this effect was demonstrated shortly after the emergence of high-intensity laser radiation. The process is coherent and gives rise to collimated radiation when induced by a collimated pump beam. In suitable birefringent nonlinear crystals the SHG process can be quite efficient. As such, it is widely used to generate new frequencies of light in conjunction with high intensity lasers. The SHG process is, however, forbidden (to a very good approximation) within the bulk of many materials. These are all materials exhibiting a center of symmetry (inversion or centrosymmetric materials). Centrosymmetry materials include essentially all liquids and gases (because the random molecular positions therein appear similar, regardless of viewing direction) as well as essentially all elemental solids. Important examples of centrosymmetric materials for the electronics industry include silicon, germanium, most metals and silicides, and most insulators, such as (amorphous) silicon dioxide. For these materials, the SHG process is appreciable only at surfaces and interfaces where the inversion symmetry of the bulk materials is broken. SHG from these materials is then dominated by the contribution of roughly one atomic layer of the material at a surface or interface. This provides the SHG process with a sensitivity to surface and interface properties not found in other optical probes. Over the last few years the SHG process has been exploited in various scientific studies or the properties of surfaces and interfaces. Issues such as the question of the density and orientation of monolayers of adsorbed molecules have been examined. The technique has also been applied to elucidate the nature of ordering and electronic structure at surfaces under ultrahigh vacuum conditions.
A survey of scientific investigations in which this technique has been employed is provided by "Optical Second-Harmonic Generation from Semiconductor Surfaces" by T. F. Heinz et al., Published in Advances in Laser Science III, edited by A. C. Tam, J. L. Cole and W. C. Stwalley (American Institute of Physics, New York, 1988) p. 452, which is hereby incorporated by reference. Other publications which will be useful in understanding the SHG effect are: "Nonlinear Optics of Surfaces and Adsorbates" by T. F. Heinz, LBL-15255, Ph. D. Thesis, University of California, Berkeley, November, 1982; "Surface Studies by Second Harmonic Generation: The Adsorption of O.sub.2, CO, and Sodium on the Rh (111) Surface" by H. W. K. Tom et al., Physical Review Letters, Vol. 52, No. 5, January 1984, American Physical Society, pp. 348-351; "Study of Si(111) Surfaces by Second Harmonic Generation: Reconstruction and Surface Phase Transformation" by T. F. Heinz et al. Physical Review Letters, Vol. 54, No. 1, January 1985, American Physical Society, pp. 63-66; "Study of Symmetry and Disordering of Si(111)-7.times.7 Surfaces by Optical Second Harmonic Generation", by T. F. Heinz et al., J. Vac. Sci. Technol., B3(5), September/October 1985, American Vacuum Society, pp. 1467-1470; "Nonlinear Optical Study of Si Epitaxy", by T. F. Heinz et al., Mat. Res. Soc. Symp. Proc., Vol. 75, 1987, pp. 697-704, Materials Research Society; "Electronic Transitions at the CaF.sub.2 /Si(111) Interface Probed by Resonant Three-Wave-Mixing Spectroscopy" by T. F. Heinz et al., Vol. 63, No. 6, August, 1989, pp. 644-647, American Physical Society; and "Surface Studies with Optical Second Harmonic Generation", by T. F. Heinz et al., Trends in Analytic Chemistry 8, pp. 235-242, 1989, all of which are also hereby fully incorporated by reference as is the information contained in the articles noted in the extensive bibliographies of these articles concerning the SHG effect. Thus it can be seen that the scientific aspects of the SHG process have been investigated extensively and the effect is deemed to be well-understood.
To summarize the SHG effect and the accuracy it provides in characterizing surfaces and boundaries near the surface of centrosymmetric materials such as silicon, metals, and insulators, it is evident that the SHG effect cannot occur efficiently as a bulk effect in these materials because of their symmetry properties. Therefore, regardless of the depth to which a beam of illuminating radiation may penetrate into the material, the SHG radiation will arise predominantly from the asymmetry present at a surface of the material or at a boundary therein. This efficient SHG will occur only in a layer with a thickness comparable to a single atomic layer. This effect is demonstrated in the results shown in FIG. 1a concerning the oxidation of a Si(111) surface under ultrahigh vacuum conditions. In this case, the oxidation reaction proceeds only to the formation of roughly one atomic layer of oxide (approximately 0.2 nm). The dramatic change in the SHG efficiency can be clearly observed.
A second example is shown in FIG. 1b, showing the SHG radiation in arbitrary units at varying depths of deposition of amorphous silicon on crystalline silicon. Thus, it can be seen that the SHG effect is relatively pronounced based on crystal structure and boundary depth without the existence of a chemical difference between the two materials on either side of the boundary.
A third example concerning an interface is given in FIG. 2. The data show the SHG efficiency for various pump laser frequencies (photon energies) for two different samples: a silicon surface covered by an oxide layer (the materials shown in FIG. 1a), indicated by circles 230 and a silicon surface covered by a calcium fluoride overlayer, indicated by dots 210 and fitted curve 220. In this case, the depth of the overlayer remains constant while the frequency of the irradiating frequency is changed. The marked differences in response can be attributed to the different nature of the interfaces in the two cases. The marked variation of SHG response with frequency in curve 220 also shows that the effect is frequency selective for a given material and also between materials. Note especially that lower curve 230, while appearing relatively flat, is clearly shown to be similarly measurable by FIG. 1a.
As is pointed out in the above incorporated articles, and shown in FIGS. 1a, 1b and 2, also represented therein, measurement of the SHG effect can yield a substantial amount of information concerning the nature of a surface or an interface. It provides the possibility for the development of a body of empirical data characterizing boundaries and surfaces with extremely high sensitivity. It should be stated that the SHG process is capable of distinguishing not only boundaries between materials with different chemical composition, but also between materials with the same chemical composition but differing crystal structures, as, for example, between amorphous or polycrystalline and crystalline silicon, as demonstrated in FIG. 1b. The SHG effect from a boundary beneath the top of the surface will generally exhibit a strong dependence on the distance of separation. On an atomic scale, this sensitivity will arise from the perturbed structural and electronic properties of the interface as its separation from the top surface decreases to a few atomic layers. On a larger length scale other factors will become relevant. These are the efficiency of propagation of the pump radiation at the fundamental frequency to the surface and the efficiency of escape of the second harmonic radiation through the overlayer. Further, interference effects arising from the contributions of the top of the surface and from the bottom boundary of the overlayer may enhance the sensitivity of the observed SHG signal on the separation. As a consequence of these effects, the amount of second-harmonic radiation generated will generally vary strongly as a boundary and the surface of the material approach or diverge from one another.
The use of the SHG process in centrosymmetric media has to date been restricted to scientific investigations of surface and interface properties. In these scientific investigations it is possible to exercise a high degree of control over the environment. For example, the amount of ambient illumination could be reduced if required. More significantly, the studies shown in FIGS. 1a, 1b and 2 were performed under highly idealized conditions, typically under ultrahigh vacuum. No bright source of radiation as would be present in a plasma in reactive ion etching or plasma enhanced chemical vapor deposition was present. Further, the surface temperatures of the samples were generally sufficiently low that they did not emit a significant amount of visible thermal radiation. Under realistic processing conditions, this may not be the case, since temperatures approaching 1000.degree. C. are frequently encountered. It has also been found that the SHG effect in silicon grows markedly weaker with increased temperature, further compounding the problem. Given these difficulties the technique has not been considered to be even potentially suitable for measurements of etching, deposition, or other reactions under typical manufacturing conditions, notably in the presence of plasmas or high surface temperatures.
Consider then the above-noted operation of the emitter opening step in device processing. While the use of the SHG effect could potentially provide excellent results, the established approach would involve removing the wafer from the etching chamber to perform the measurements. Thus while the observation of the SHG effect could provide a resolution on the order of one atomic spacing, the realization of such accuracy in a practical device would require etching in steps of one atomic layer and checking after each step, thereby multiplying the complexity of the manufacturing process enormously.
In summary, the capability of the measurement technology and the present state of semiconductor manufacturing technology indicate a need for some arrangement whereby the full capabilities of both can be simultaneously realized. Specifically, while observation of the SHG effect can provide resolution of the position of a boundary to one atomic spacing and current techniques of material deposition and removal can ideally provide an equivalent accuracy, such capabilities cannot practically be implemented unless the SHG effect can be monitored during the course of such material deposition, removal or modification in order to observe and control it.